var keythereum = require("keythereum");
// Specify a data directory (optional; defaults to ~/.ethereum)
var datadir = "E:\\geth\\testNet";

// Synchronous
var keyObject = keythereum.importFromFile("0x10a5fa2fFF03438941F262A660B4cD073524C205", datadir);
var privateKey = keythereum.recover("123456", keyObject);
console.log(privateKey.toString('hex'))//0xc276bd04daeb14439d5e54b463b970f85515bfc26fb5440b7939d8df8f40c4dc